#include<iostream>
#include<iomanip>
#include<algorithm>
#include<cmath>
using namespace std;
double a, b, c;
int main()
{
	cin >> a >> b >> c;
	double p = (a + b + c) * 1.0 / 2;
	double s = p * (p - a) * (p - b) * (p - c);
	double area = sqrt(s);
	cout << fixed << setprecision(1) << area << endl;
	return 0;
}